1. Field of the Invention
The present invention relates to an optical modulation element and, more particularly, to an optical modulation element wherein a grating and a material having a variable refractive refraction index are combined to control the refraction index of the variable refraction index material, thereby causing desired diffraction of incident light.
2. Related Background Art
A conventional well-known optical modulation element is a so-called TN (twisted nematic)-type liquid crystal display element which consists of a pair of polarizing plates arranged such that their polarizing directions are perpendicular to each other, and an element which is arranged between the pair of polarizing plates and obtained by sealing a liquid crystal on opposing substrate surfaces of a pair of transparent substrates and making orthogonal orientation treatment each other. The orientation state of the liquid crystal is switched between a twisted state and a state perpendicular to the substrate surface, thereby modulating incident light. A display element of this type is widely used since it has simple constructions and be able to be easily driven. However, since this display element transmits and shields light by utilizing two polarizing plates, its transmittance rate during extinguishing, i.e., during light transmission is poor. Therefore, this element is not a preferable optical modulation element in terms of light utilization efficiency.
A so-called guest-host mode liquid crystal display element in which dyes are mixed in liquid crystal molecules is known as another display element of the same type which utilizes a liquid crystal. However, because of the presence of the dyes, this display element has a transmittance during extinguishing of as low as about 75% at maximum.
Japanese Patent examined Publication No. 3928/1978, U.S. Pat. No. 4,251,137, and so on, disclose a display element or a variable subtractive process filter element wherein a reflective or transmitting phase diffraction grating is combined with a liquid crystal. The elements disclosed in these articles have good light utilization efficiency. However, the element disclosed in Japanese Patent examined Publication No. 3928/1978 has a mere ornamental effect and is not satisfactory as an optical modulation element for transmitting or shielding light. According to the variable subtractive process filter disclosed in U.S. Pat. No. 4,251,137, diffraction gratings are formed on a pair of opposing substrate surfaces such that their orientation directions are perpendicular to each other. A liquid crystal is filled between these substrates to control the orientation of the liquid crystal molecules, thereby changing the refraction index of the filter element. When the difference in refraction index between the material constituting the diffraction gratings and the liquid crystal is changed, the spectral transmittance characteristics can be changed. Therefore, this filter has a high light utilization efficiency and has high performance as a variable subtractive process filter. This variable subtractive process filter mainly transmits and displays three primary colors of R, G, and B, or their complementary colors C, M and Y. However, for example, during extinguishing, i.e., when incident light is to be totally transmitted, extinguishing, i.e., total transmission is impossible since the wavelength dependency of the refractive index of the liquid crystal and that of the material constituting the diffraction grating are not considered at all. As a result, the contrast is degraded.
With an element constitution of this type consisting of diffraction gratings and the variable refraction index material, when the refraction index of the variable refraction index material is to be controlled, total transmission and extinguishing of incident light are difficult to obtain since no definite threshold value at which the refraction index changes exists and the change is very slow. Also, multiplex driving cannot be performed when a plurality of elements of this type are used to provide a display apparatus.